This invention relates to a jaw assembly for a stretch former, such as a stretch-wrap machine. Stretch-wrap forming is a process of forming contoured metal parts, or workpieces, by pulling and wrapping extrusions or formed workpieces around a die. Other actions may be performed on the workpiece at the same time. Stretch-wrap machines tightly clamp the opposing ends of the workpiece in powerful jaws while pivotally-mounted tension cylinders on which the jaws are mounted wrap the workpiece around the curved surface of the die in a combined stretching and bending action.
During the stretch-forming process, the stress on the workpiece in the region nearest the convex side becomes so great that the metal reaches a state of semi-plasticity. This occurs by stretching the workpiece beyond its yield point. Metal structures undergoing such deformation, upon release of the tension on it, will remain permanently deformed. This permanent deformation of the metal causes the workpiece to remain in its desired shape into which it was formed during the stretch-wrap process.
The workpiece is held in the jaws by means of inserts. There are usually several insert segments which collectively make up the insert. These insert segments fit together and define at their adjoining, axially-extending surfaces a profile which corresponds to the profile, i.e., the cross-section, of the workpiece to be formed. The insert segments are placed within an insert chamber in the nose piece of the jaw. They are retained in the jaw, without attachment to each other, by a retaining ring of reduced diameter fitted to the face of the nose piece. The insert is mounted for sliding movement to a tapered insert sleeve. The taper on the insert sleeve is oriented so that the pull of the opposite tension cylinder wedges the insert ever more tightly against the insert sleeve, creating intense clamping pressures on the ends of the workpiece. When the opposed tension cylinders release tension on the workpiece, the pressure is likewise reduced on the workpiece by the jaws. The insert is then retracted by a hydraulic cylinder assembly to loosen the insert segments, so that the workpiece can be removed and a new workpiece inserted.
Whenever a workpiece having different profiles is to be formed, the insert must be changed. In prior art devices, this requires that the nose piece be partially disassembled, and the insert segments manipulated to avoid interference from the insert actuator to which the insert is mounted as they are removed. The insert actuator has an enlarged shoulder around which the insert segments fit. In order to remove the insert segments, the segments are retracted so that the taper loosens the insert segments. In prior art jaws, this distance is insufficient to permit removal of the insert segments without carefully manoeuvering the segments away from the insert actuator.
This is a difficult task, since the area inside the nose piece is quite confined, and the insert segments are very heavy and difficult to handle. Injury can occur, since the hands of the operator must be inserted into the nose piece. Placement of the new insert segments is likewise difficult, since the segments must be carefully fitted into the nose piece and into locking contact with the insert actuator.
This invention increases the ease with which the insert segments which make up the insert can be removed and replaced by providing for retraction of the insert actuator well beyond the workpiece remove position.